Alternatives for the tetras include smaller rasboras (harlequins, mosquito, micro etc), danios (that can be a risk, though), or gentle barbs, such as cherries. Alternatively, livebearers such as platies, guppies or Endlers (my favourite) would do, but only 4 platies or guppies as they have a higher bioload than little tetras.

I'd be careful about guppys tho, with the betta. Your betta might see the flashy male guppys as a threat, and might attack them. It depends on his personality. However, my female betta does great with the 4 male guppys she's with.

What I did to test my betta is hold a mirror up to him. From my experience, if it takes him forever to flare out, or if he swims away, he's not verry aggressive. I also put in 5 neon tetras with him in a small tank, and he ignored them, and neons are pretty noticable! I'm also gonna test my Gyarados by putting him in the small tank with the guppys, just to see how he does. If he starts to get aggressive, I can pull him out quickly.

my Captain is very calm and attitude free. he hardly flares. Red on the other hand. He is going to be put in a bigger tank soon but I don't think he is going to have any buddy's. He flares at everything and everyone except me.